Muscles and Muscle Tissue: Part A

PowerPoint Lecture Slides prepared by Janice Meeking, Mount Royal College CHAPTER 9 Muscles and Muscle Tissue: Part A

Warm Up 12/12/16 Describe the major differences between cardiac, skeletal and smooth muscle in terms of appearance, control, function, and location!

Three Types of Muscle Tissue 1. Skeletal muscle tissue: Attached to bones and skin Striated Voluntary (i.e., conscious control) Powerful Primary topic of this chapter

Three Types of Muscle Tissue 2. Cardiac muscle tissue: Only in the heart Striated Involuntary More details in Chapter 18

Three Types of Muscle Tissue 3. Smooth muscle tissue: In the walls of hollow organs, e.g., stomach, urinary bladder, and airways Not striated Involuntary More details later in this chapter

Table 9.3

Special Characteristics of Muscle Tissue Excitability (responsiveness or irritability): ability to receive and respond to stimuli Contractility: ability to shorten when stimulated Extensibility: ability to be stretched Elasticity: ability to recoil to resting length

Muscle Functions 1. Movement of bones or fluids (e.g., blood) 2. Maintaining posture and body position 3. Stabilizing joints 4. Heat generation (especially skeletal muscle)

Skeletal Muscle Each muscle is served by one artery, one nerve, and one or more veins

Skeletal Muscle Connective tissue sheaths of skeletal muscle: Epimysium: dense regular connective tissue surrounding entire muscle Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers) Endomysium: fine areolar connective tissue surrounding each muscle fiber

Bone Epimysiu m Tendon (b ) Perimysiu (a m ) Fascicl e Epimysiu m Perimysiu m Endomysiu m Muscle fiber in middle of a fascicle Blood Fascicle vessel (wrapped by perimysium) Endomysium (between individual muscle fibers) Muscle fiber Figure 9.1

Skeletal Muscle: Attachments Muscles attach: Directly epimysium of muscle is fused to the periosteum of bone or perichondrium of cartilage Indirectly connective tissue wrappings extend beyond the muscle as a ropelike tendon or sheetlike aponeurosis

Table 9.1

Microscopic Anatomy of a Skeletal Muscle Fiber Cylindrical cell 10 to 100 μm in diameter, up to 30 cm long Multiple peripheral nuclei Many mitochondria Glycosomes for glycogen storage, myoglobin for O2 storage Also contain myofibrils, sarcoplasmic reticulum, and T tubules

Myofibrils Densely packed, rodlike elements ~80% of cell volume Exhibit striations: perfectly aligned repeating series of dark A bands and light I bands

Sarcolemm a Mitochondrion Myofibri l Dark A band Light I Nucleus bandof a muscle fiber showing the myofibrils. (b) Diagram of part One myofibril is extended afrom the cut end of the fiber.

Sarcomere Smallest contractile unit (functional unit) of a muscle fiber The region of a myofibril between two successive Z discs Composed of thick and thin myofilaments made of contractile proteins

Features of a Sarcomere Thick filaments: run the entire length of an A band Thin filaments: run the length of the I band and partway into the A band Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another H zone: lighter midregion where filaments do not overlap M line: line of protein myomesin that holds adjacent thick filaments together

Thin (actin) filament Thick (myosin) filament Z disc H zone Z disc A band I band M line Sarcomer e (c Small part of one myofibril enlarged to show the myofilaments ) responsible for the banding pattern. Each sarcomere extends from one Z disc to the next. Z disc I band Sarcomer e M line Z disc Thin (actin) filament Elastic (titin) filaments Thick (myosin ) filament (d Enlargement of one sarcomere (sectioned lengthwise). Notice the ) myosin heads on the thick filaments. Figure 9.2c, d

Ultrastructure of Thick Filament Composed of the protein myosin Myosin tails contain: 2 interwoven, heavy polypeptide chains Myosin heads contain: 2 smaller, light polypeptide chains that act as cross bridges during contraction Binding sites for actin of thin filaments Binding sites for ATP ATPase enzymes

Ultrastructure of Thin Filament Twisted double strand of fibrous protein F actin F actin consists of G (globular) actin subunits G actin bears active sites for myosin head attachment during contraction Tropomyosin and troponin: regulatory proteins bound to actin

Longitudinal section of filaments within one sarcomere of a myofibril Thick filament Thin In the center of the sarcomere, the thick filament filaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap. Thick filament Thin filament Each thick filament consists of many A thin filament consists of two strands myosin molecules whose heads protrude of actin subunits twisted into a helix at opposite ends of the filament. plus two types of regulatory proteins (troponin and tropomyosin). Portion of a thick filament Portion of a thin Myosin filament head Tropomyosi Troponi Acti n n n Actin-binding sites ATPbindin g site Head s Flexible hinge region Myosin molecule Tai l Active sites for myosin attachment Actin subunits Actin subunits Figure 9.3

Sarcoplasmic Reticulum (SR) Network of smooth endoplasmic reticulum surrounding each myofibril Pairs of terminal cisternae form perpendicular cross channels Functions in the regulation of intracellular Ca2+ levels

T Tubules Continuous with the sarcolemma Penetrate the cell s interior at each A band I band junction Associate with the paired terminal cisternae to form triads that encircle each sarcomere

Part of a skeletal muscle fiber (cell) I band Z Myofibril disc Sarcolemm a A band I band H zone M line Z disc Sarcolemm a Triad: T Terminal tubule cisternae of the SR (2) Tubules of the SR Myofibril Mitochondria s Figure 9.5

Triad Relationships T tubules conduct impulses deep into muscle fiber Integral proteins protrude into the intermembrane space from T tubule and SR cisternae membranes T tubule proteins: voltage sensors SR foot proteins: gated channels that regulate Ca2+ release from the SR cisternae

9A-2 Notes:

Warm Up! Explain the role and function of the T tubules and sarcoplasmic reticulum in relation to muscle contraction.

Contraction The generation of force Does not necessarily cause shortening of the fiber Shortening occurs when tension generated by cross bridges on the thin filaments exceeds forces opposing shortening

Sliding Filament Model of Contraction In the relaxed state, thin and thick filaments overlap only slightly During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens

Z Z H A I I 1 Fully relaxed sarcomere of a muscle fiber Z I Z A I 2 Fully contracted sarcomere of a muscle fiber Figure 9.6

Requirements for Skeletal Muscle Contraction 1. Activation: neural stimulation at a neuromuscular junction 2. Excitation-contraction coupling: Generation and propagation of an action potential along the sarcolemma Final trigger: a brief rise in intracellular Ca2+ levels

Events at the Neuromuscular Junction Skeletal muscles are stimulated by somatic motor neurons Axons of motor neurons travel from the central nervous system via nerves to skeletal muscles Each axon forms several branches as it enters a muscle Each axon ending forms a neuromuscular junction with a single muscle fiber

Action potential (AP) Myelinated axon of motor neuron Axon terminal of neuromuscular junction Nucleu s Sarcolemma of the muscle fiber 1 Action potential arrives at axon terminal of motor neuron. 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. Ca2+ Ca2 + Axon terminal of motor neuron Synaptic vesicle containing ACh Mitochondrion Synaptic cleft Fusing synaptic vesicles Figure 9.8

Neuromuscular Junction Situated midway along the length of a muscle fiber Axon terminal and muscle fiber are separated by a gel-filled space called the synaptic cleft Synaptic vesicles of axon terminal contain the neurotransmitter acetylcholine (ACh) Junctional folds of the sarcolemma contain ACh receptors

Events at the Neuromuscular Junction Nerve impulse arrives at axon terminal ACh is released and binds with receptors on the sarcolemma Electrical events lead to the generation of an action potential PL AY A&P Flix : Events at the Neuromuscular Junction

Myelinated axon of motor neuron Axon terminal of neuromuscular junction Sarcolemma of the muscle fiber Action potential (AP) Nucleu s 1 Action potential arrives at axon terminal of motor neuron. 2+ 2 Voltage-gated Ca channels Ca2+ Ca2+ open and Ca2+ enters the axon terminal. Axon of motor terminal neuron 3 Ca2+ entry causes some Fusing synaptic vesicle s synaptic vesicles to release their contents (acetylcholine) by exocytosis. 4 Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. AC h 5 ACh binding opens ion Na+ K+ channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. 6 ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase. Ach Degraded ACh Na+ Acetylcholinesterase Synaptic vesicle containing ACh Mitochondrion Synaptic cleft Junctional folds of sarcolemm a Sarcoplasm of muscle fiber Postsynaptic membrane ion channel opens; ions pass. Postsynaptic membrane ion channel closed; ions cannot pass. K+ Figure 9.8

Destruction of Acetylcholine ACh effects are quickly terminated by the enzyme acetylcholinesterase Prevents continued muscle fiber contraction in the absence of additional stimulation

Events in Generation of an Action Potential 1. Local depolarization (end plate potential): ACh binding opens chemically (ligand) gated ion channels Simultaneous diffusion of Na+ (inward) and K+ (outward) More Na+ diffuses, so the interior of the sarcolemma becomes less negative Local depolarization end plate potential

Events in Generation of an Action Potential 2. Generation and propagation of an action potential: End plate potential spreads to adjacent membrane areas Voltage-gated Na+ channels open Na+ influx decreases the membrane voltage toward a critical threshold If threshold is reached, an action potential is generated

Events in Generation of an Action Potential Local depolarization wave continues to spread, changing the permeability of the sarcolemma Voltage-regulated Na+ channels open in the adjacent patch, causing it to depolarize to threshold

Events in Generation of an Action Potential 3. Repolarization: Na+ channels close and voltage-gated K+ channels open K+ efflux rapidly restores the resting polarity Fiber cannot be stimulated and is in a refractory period until repolarization is complete Ionic conditions of the resting state are restored by the Na+-K+ pump

Axon terminal Open Na+ Channel Na Synaptic cleft Closed K+ Channel + ACh t io n za + of de po + + l ari ACh Na K Na K + e Wav 1 Local depolarization: generation of the end plate potential on the sarcolemma K ++ ++ + + + Action potential + + +++ + 2 Generation and propagation of the action potential (AP) Closed Na+ Channel Na Open K+ Channel + K Sarcoplasm of muscle fiber 3 Repolarization + Figure 9.9

Axon terminal Open Na+ Channel Na Synapti c cleft + n + K ++ ++ + + + Action potential + + +++ + l ar + of d ep o + K tio AC h Na K Na + iza AC h Closed K+ Channel ve Wa 1 Local depolarization: generation of the end plate potential on the sarcolemma Sarcoplasm of muscle fiber Figure 9.9, step 1

Axon terminal Open Na+ Channel Na Synapti c cleft + n + K l ar + ++ ++ + + + Action potential + + +++ + 2 Generation and propagation of the action potential (AP) of d ep o + K tio AC h Na K Na + iza AC h Closed K+ Channel ve Wa 1 Local depolarization: generation of the end plate potential on the sarcolemma Sarcoplasm of muscle fiber Figure 9.9, step 2

Open K+ Channel Closed Na+ Channel Na + K + 3 Repolarization Figure 9.9, step 3

Axon terminal Open Na+ Channel Na Synapti c cleft t io n + + K ++ ++ + + + Action potential + + +++ + 2 Generation and propagation of the action potential (AP) of de po + Na K + za AC h Na K + l ari AC h Closed K+ Channel e Wav 1 Local depolarization: generation of the end plate potential on the sarcolemma Closed Na+ Na Channel + Open K+ Channe l K Sarcoplasm of muscle fiber 3 Repolarization + Figure 9.9

Depolarizatio n due to Na+ entry Na+ channels open Na+ channels close, K+ channels open Repolarizatio n due to K+ exit Threshold K+ channels close Figure 9.10

Excitation-Contraction (E-C) Coupling Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the myofilaments Latent period: Time when E-C coupling events occur Time between AP initiation and the beginning of contraction

Events of Excitation-Contraction (E-C) Coupling AP is propagated along sarcolemma to T tubules Voltage-sensitive proteins stimulate Ca2+ release from SR Ca2+ is necessary for contraction

Setting the stage Axon terminal of motor neuron Action Synaptic cleft potential ACh is generated Sarcolemma Terminal cisterna of SR Muscle fiber Ca2+ Triad One sarcomere Figure 9.11, step 1

Steps in E-C Coupling: Sarcolemma Voltage-sensitive tubule protein T tubule 1 Action potential is propagated along the sarcolemma and down the T tubules. Ca2+ release channe l 2 Calcium ions are released. Terminal cisterna of SR Ca2+ Actin Troponi n Ca2+ Tropomyosin blocking active sites Myosi n 3 Calcium binds to troponin and removes the blocking action of tropomyosin. Active sites exposed and ready for myosin binding Myosi n cross bridge 4 Contraction begins The aftermath Figure 9.11, step 2

1 Action potential is propagated along the sarcolemma and Steps in down E-C Coupling: the T tubules. Sarcolemma Voltage-sensitive T tubule tubule protein Ca2+ release channel Terminal cisterna of SR Ca2+ Figure 9.11, step 3

Actin Ca2+ Troponi n Tropomyosin blocking active sites Myosin The aftermath Figure 9.11, step 5

Actin Ca2+ Troponi n Tropomyosin blocking active sites Myosin 3 Calcium binds to troponin and removes the blocking action of tropomyosin. Active sites exposed and ready for myosin binding The aftermath Figure 9.11, step 6

Actin Ca2+ Troponi n Tropomyosin blocking active sites Myosin 3 Calcium binds to Active sites exposed and ready for myosin binding troponin and removes the blocking action of tropomyosin. 4 Contraction begins Myosin cross bridge The aftermath Figure 9.11, step 7

Role of Calcium (Ca2+) in Contraction At low intracellular Ca2+ concentration: Tropomyosin blocks the active sites on actin Myosin heads cannot attach to actin Muscle fiber relaxes

Role of Calcium (Ca2+) in Contraction At higher intracellular Ca2+ concentrations: Ca2+ binds to troponin Troponin changes shape and moves tropomyosin away from active sites Events of the cross bridge cycle occur When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends

Cross Bridge Cycle Continues as long as the Ca2+ signal and adequate ATP are present Cross bridge formation high-energy myosin head attaches to thin filament Working (power) stroke myosin head pivots and pulls thin filament toward M line

Cross Bridge Cycle Cross bridge detachment ATP attaches to myosin head and the cross bridge detaches Cocking of the myosin head energy from hydrolysis of ATP cocks the myosin head into the high-energy state

Thin filament Actin Myosin cross bridge Ca 2+ ADP Pi Thick filamen Myosi t n 1 Cross bridge formation. ADP ADP Pi Pi ATP hydrolysi s 2 The power (working) stroke. 4 Cocking of myosin head. AT P AT P 3 Cross bridge detachment. Figure 9.12

Actin Ca2 Thin filament + Myosin cross bridge AD PP i Thick filament Myosi n 1 Cross bridge formation. Figure 9.12, step 1

AD P P i 2 The power (working) stroke. Figure 9.12, step 3

ATP 3 Cross bridge detachment. Figure 9.12, step 4

AD PP i ATP hydrolysi s 4 Cocking of myosin head. Figure 9.12, step 5

Steps in E-C Coupling - Storytelling! 1. Fill in the blanks to the steps of the E-C story that I wrote out for you! 2. Define the terms in the word bank provided. 3. Retell the story by creating your own captions on the pictures provided. Be sure to label all * structures as well!

To instigate the process at the Neuromuscular Junction. Neurotransmitters are released from the axon terminal. As they diffuse across the synaptic cleft, they attach to Acetylcholine (ACh) receptors on the sarcolemma.

Step 1 + Net entry of Na initiates an Action Potential which is propagated along the sarcolemma and down the T Tubules.

Step 2 The action potential in the T tubule activates voltage-sensitive receptors which in turn trigger Ca2+ release from the terminal cisternae of the Sarcoplasmic Reticulum in the cytosol.

Step 3 Calcium ions bind to troponin; this changes shape, removing the blocking action of tropomyosin therefore leave the actin active sites exposed.

Step 4 Contraction! (The Cross Bridge Cycle***) Myosin heads alternately attach to actin and detach, pulling the Actin filaments toward the center of the sarcomere. Powered by ATP hydrolysis.

Step 4 broken down 4.1 *** Myosin head attached to the actin myofilament, forming the cross bridge.

4.2*** Inorganic Phosphate (Pi) generated in the previous contraction cycle is released, initiating the power (working) stroke. The myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M-Line. Then ADP is released.

4.3*** A new ATP attached to the myosin head, the link between myosin and actin weakens, and the cross bridge detaches.

4.4*** As ATP split into ADP and Pi, the myosin head is energized (cocked into the high-energy conformation) Ready to start over!

Step 5 Removal of Ca 2+ by active transport into the sarcoplasmic reticulum (SR) after the action potential ends.

Step 6 Tropomyosin blockage is restored, blocking the myosin binding sites on actin. Contraction ends and the muscle fiber relaxes!!

Warm Up 1/9/17 Through what process is ATP made?! With oxygen = Without oxygen = (Where does this process occur?)

What are the 6 Steps of EC Coupling?! Remember that step 4 is our dance! (the cross bridge cycle!) - broken down into 4.1, 2, 3 and 4!

mitochondria 1 Axon terminal Sodium rushes into the cell, sending an action potential down the length of the cell membrane (sarcolemma) and into the cell via T Tubules. Synaptic vesicle ACh Receptor Acetylcholine (ACh) Sarcolemma

Sarcoplasmic Reticulum (SR) Calcium T Tubule 2 Voltage gated receptor T tubules stimulate calcium release from the SR

3 Calcium binds to troponin, unlocking the tropomyosin gate and exposing the myosin binding sites Myosin binding site Actin Myosin head Troponin Tropomyosin Myosin

4 Contraction begins! Actin is pulled towards the center of the sarcomere by myosin. (Also known as the Cross Bridge Cycle) Let s pr acti our dan ce ce!

Sarcoplasmic Reticulum (SR) 5 Calcium Calcium is removed from the cytosol and stored back in the Sarcoplasmic Reticulum for future use!

6 Contraction ends! Tropomyosin gate and troponin lock are restored to their original positions, blocking the myosin sites on actin.